The present invention relates to a continuous multistage distillation process for separating a partially water soluble component from a slurry containing an insoluble solid.
Such separation is required in the production of some inorganic pigments where the product pigment may be synthesized in the form of a slurry containing water and/or an organic compound, by-products and/or unreacted materials as well as the desired pigment. Recovery of the desired pigment in dry form requires removal of such organic compounds, by-products, and unreacted materials. The degree of difficulty in removing such materials necessarily depends upon the specific process and the particular materials present in the slurry.
An example of considerable economic relevance is the aniline process for manufacturing iron oxide color pigments. With a worldwide production capacity of 1.1 billion pounds per year, synthetic iron oxides are the largest volume color pigment in use. Concrete products, coatings and pigments for plastics are among the major markets.
Iron oxide color pigments are generally manufactured by one of four basic processes: (1) the Copperas process, (2) the precipitation process, (3) the Penniman process and (4) the aniline or so-called Bechamp process. In each of the first three processes, iron salts and, with the exception of the precipitation process, stamping scrap iron are used to generate iron oxides by roasting or by precipitation in an aqueous environment. Separation of the product iron oxide from its aqueous environment is readily accomplished.
In the aniline process (described e.g., in U.S. Pat. No. 4,145,228), however, mononitrobenzene is reacted with cast iron scrap in the presence of iron (II) and/or aluminum chloride solutions to produce both aniline and iron oxide. Separation of the crude aniline from the iron oxide is not as easily achieved as separation of iron oxide from water. In fact, such separation is commonly carried out in two stages. In the initial separation stage, crude aniline and a slurry in which iron oxide, water and by-products are present, are obtained from the reaction mixture. In a second step, the iron oxide is separated from the water and the inorganic by-products via conventional methods. Several methods for separation of the aniline from the slurry are known. Examples of such separation techniques are (1) vacuum distillation, (2) filtration, (3) steam distillation and (4) a combination of decantation and steam distillation.
Vacuum distillation of the reaction mixture generates a dry sludge having a very high residual aniline content. This residual aniline content can be reduced to an acceptable level only by subsequent distillation with live steam.
Filtration of the reaction mixture, e.g. through large metal screen filter boxes, yields an iron oxide-containing filter cake. This filter cake must then be washed with hot water or stripped with steam and dried. This separation technique has the disadvantage of reducing the quality of the iron oxide pigment and is difficult to control from an industrial hygiene viewpoint.
Steam distillation results in the most complete separation of aniline and iron oxide without detrimentally affecting the quality of the iron oxide pigment. In this separation method, live steam is introduced into the reactor in which the aniline and iron oxide are present to strip the volatile aniline from the reaction mixture. The resultant aniline-water distillate must then be decanted to separate the aniline and water layers. The aniline-containing water fraction and the aniline fraction must each be purified by additional distillation. The energy cost of such a distillation of the reaction mixture becomes prohibitive unless distillation is supplemented with a mechanical method of separation such as sedimentation.
Sedimentation may be accomplished by discontinuing the agitation of the reaction mixture after the reduction reaction has been completed. A thick slurry layer and a supernatant-aniline layer form very shortly after the agitation has stopped. The aniline layer may be readily siphoned off leaving a slurry made up of residual aniline, water, and iron oxide in the reactor. Live steam may then be introduced into the reactor and agitation of the mixture resumed. The steam distillation is continued until the aniline concentration in the mixture is reduced to approximately 500 to 1000 ppm. Reduction of the aniline content below this range is possible but not economically feasible because the cost of the steam required would far outweigh the benefits of product recovery and lower residual aniline content in downstream washing steps.
After distillation via live steam has been completed, the remaining crude iron oxide slurry is classified and washed to remove water soluble salts and foreign materials. The residue and the wash water from this treatment contain aniline and must therefore be processed, e.g. in a biological treatment facility, prior to disposal or recycling.
It would, therefore, be advantageous to improve the separation technology for such a mixture in a manner which would be more efficient and allow for a more complete separation than the known processes.